EXP. 35 A & B OXIDATION-REDUCTION SCHEME: BORNEOL - CAMPHOR - ISOBORNEOL
LEARNING OBJECTIVES: To illustrate the concepts of oxidation and reduction in organic chemistry, to illustrate the stereochemical effects of these reactions in certain systems, to use IR spectroscopy to characterize diastereomers and monitor reactions.
QUIZ PREPARATION: Recitation notes and readings assigned in the syllabus.
OXIDATION AND REDUCTION IN ORGANIC CHEMISTRY
The concepts of oxidation and reduction in chemistry are fundamentally related to the loss and gain of electrons, respectively. For a specific atom, oxidation brings about an increase in the oxidation number, whereas reduction does the opposite. This is of course true in organic reactions as well, but it is frequently less clear where the gain or loss of electrons is taking place. For this reason, the concepts of oxidation and reduction are addressed from a different perspective in organic chemistry.
Since carbon is the most important element in organic chemistry, we can define oxidation as a process whereby carbon gains bonds to more electronegative atoms. These can be any atoms more electronegative than carbon, such as chlorine or sulphur, but the most commonly seen in organic oxidations is oxygen. The more bonds to oxygen, the higher the oxidation state of the carbon involved. Examples of oxidation reactions are those that convert alkanes into alcohols, or alcohols into carbonyl compounds. The functional groups that contain the most highly oxidized form of carbon in organic compounds are the carboxylic acids and their derivatives.
Reduction is the opposite of oxidation. We can define reduction as a process whereby carbon adds bonds to less electronegative atoms. Again, the most common of these atoms is hydrogen. Therefore, any reactions that cause carbon to add more bonds to hydrogen can be referred to as reductions. Examples are the conversion of carbonyl compounds into alcohols, or even alkanes. Obviously, alkanes represent the most reduced (least oxidized) type of organic compounds possible.
The following scheme shows the most common functional groups involved in organic oxidations and reductions. Any chemical conversions that proceed to the right can be classified as oxidations, and any reactions that proceed to the left can be classified as reductions.
H H O O R C R R C OH R C H R C OH H H carbonl compound Alkane alcohol (aldehyde or ketone) carboxylic acid
Increasing oxidation state of carbon (oxidation)
Decreasing oxidation state of carbon (reduction) THE REACTIONS OF EXPERIMENT 35
Refer to pages 289 - 290 of your textbook.
It is clear now how the oxidation-reduction scheme of p. 289 is being used here to illustrate the concept experimentally. An alcohol (borneol) is being oxidized into a ketone (camphor). A subsequent reduction takes us back to another alcohol (isoborneol), which is an isomeric form of the original.
The oxidizing agent in the first step is sodium hypochlorite, which is present in commercial bleach as an aqueous solution. In fact, the reaction can be performed using bleach, but it must be relatively fresh. Sodium hypochlorite decomposes in water slowly, and using old bleach could introduce a source of error in the experiment.
The reducing agent in the second step is sodium borohydride, which will be used in alcoholic solution. Again, sodium borohydride reacts with alcohols slowly. Therefore it is important that the solution be freshly prepared. If you monitor your reactions by IR you’ll have a good indicator of the effectiveness of the conversions, and therefore of the quality of the reagents used. Ineffective reagents can result in incomplete conversions, which will show the presence of starting material in the IR spectrum of products.
STEREOCHEMISTRY OF THE REACTION
The diagram at the bottom of p. 290 shows the stereochemical detail of the mechanism of the reactions. It is important to understand the meaning of the endo and exo terminology. This terminology is used in reference to bicyclic systems such as the norbornane molecule shown below. The rigidity of these systems makes hydrogens (a) and (b) stereochemically nonequivalent. The exo hydrogen points outward from the ring, and the endo hydrogen points downward, opposite to the bridge, as shown.
bridge
Norbornane Ha exo hydrogens Ha Hb Hb endo hydrogens
If we apply this terminology to the alcohols borneol and isoborneol, we can see that they are not interconvertible, and therefore they are isomers. What kind of isomers are they? The fact that the carbon bearing the hydroxl group is chiral, and that there are other chiral centers in each of these molecules, makes these alcohols diastereomers. An instance showing some of these chiral centers (but not all, for clarity) is seen below.
H3C CH3 H3C CH3
H OH R R S R H3C H3C OH H
Borneol Isoborneol
Diasteromers The next question is, since the starting material (borneol) can exist as a racemic mixture (see below), is it being used as such in the lab, or is it a pure enantiomer? The answer is easy. Pure enantiomers are expensive and difficult to prepare. Therefore, the reagent used in the lab is a racemic mixture. As extra information, the configuration assignment for one of the carbons is also shown.
H3C 2 CH3 H3C CH3
3 H H R 1 S R S H3C CH3 OH OH
Racemic borneol (enantiomers)
Now, how does the presence of enantiomers in the starting material affect the final outcome of these conversions? Each step of the reaction effects the same conversion on both enantiomers, therefore, each product from each step is obtained as an enantiomeric mixture, including the final product.
H3C 2 CH3 H3C CH3
3 H H R 1 S R S H3C CH3 OH OH
Racemic borneol (enantiomers)
OX
H3C CH3 H3C CH3 H3C CH3 H3C CH3 RED
OH HO R S R R S S H3C CH3 H3C CH3 O O H H
Enantiomeric camphor Racemic Isoborneol (enantiomers) EXP. 28 A, B PRACTICAL CONSIDERATIONS
There are no special considerations or procedural changes for this experiment. However, pay special attention to the following:
(a) Please cap all reagent bottles immediately after use. As mentioned before, these reagents are sensitive to moisture and other sources of contamination.
(b) The experiment takes two lab periods, and taking melting points and IR spectra for each product are requirements. Do not attempt to finish in one period. The instructors are under no obligation to wait if you start running behind.
(c) When examining your products by IR, pay special attention to the fingerprint region. This is where most of the differences between the two isomeric alcohols are.